<template>
  <my-scroll class="listview" :data="data" :listenScroll="listenScroll" :probeType="probeType" @scroll="scroll" ref="listview">         <!-- 监听scroll里面派发出来的事件 -->
    <ul>
      <li v-for="(group,index) in data" class="list-group" ref="listGroup">
        <h2 class="list-group-title">{{group.title}}</h2>
        <ul>
          <li @click="selectItem(item)" v-for="(item,index) in group.items" class="list-group-item">
            <img v-lazy="item.avatar" alt="img" class="avatar">
            <span class="name">{{item.name}}</span>
          </li>
        </ul>
      </li>
    </ul>
    <div class="list-shortcut" @touchstart="onShortcutTouchStart" @touchmove.stop.prevent="onShortcutTouchMove">
      <ul>
        <li v-for="(item,index) in shortcutList" class="item" :class="{'current': currentIndex===index}" :data-index="index">{{item}}</li>
      </ul>
    </div>
    <div class="list-fixed" ref="fixed" v-show="fixedTitle">
      <div class="fixed-title">{{fixedTitle}}</div>
    </div>
    <div v-show="!data.length" class="loading-container">
      <loading></loading>
    </div>
  </my-scroll>
</template>

<script>
  import Scroll from 'base/scroll/scroll'
  import {getData} from 'common/js/dom'
  import Loading from 'base/loading/loading'

  const TITLE_HEIGHT = 30
  const ANCHOR_HEIGHT = 18                           // 每一个li的高度

  export default {
    created () {
      this.touch = {}                                 // 这里不把他添加到data(){return {touch: {}}}。是因为不需要去监听touch的变化。不用给他添加get，set方法
      this.listenScroll = true
      this.listHeight = []
      this.probeType = 3                         // 允许监听快速滚动的
    },
    data () {
      return {
        scrollY: -1,
        currentIndex: 0,
        diff: -1
      }
    },
    props: {
      data: {
        type: Array,
        default: []
      }
    },
    computed: {
      shortcutList () {
        return this.data.map((group) => {              // 数组的map方法
          return group.title.substring(0, 1)
        })
      },
      fixedTitle () {
        if (this.scrollY > 0) {
          return ''
        }
        return this.data[this.currentIndex] ? this.data[this.currentIndex].title : ''
      }
    },
    components: {
      'my-scroll': Scroll,
      Loading                                       // 'my-loading': Loading    用法此处报错。不确实原因是否因为被包裹在scroll组件内
    },
    methods: {
      selectItem (item) {
        this.$emit('select', item)                              // listview是基础组件，不关心业务逻辑。仅仅只是处理。我被点击了，并传递点击它的对象
      },
      onShortcutTouchStart (e) {
        let anchorIndex = getData(e.target, 'index')
        let firstTouch = e.touches[0]
        this.touch.y1 = firstTouch.pageY
        this.touch.anchorIndex = anchorIndex
        this._scrollTo(anchorIndex)
      },
      onShortcutTouchMove (e) {
        let firstTouch = e.touches[0]
        this.touch.y2 = firstTouch.pageY
        let delta = (this.touch.y2 - this.touch.y1) / ANCHOR_HEIGHT | 0                  // y轴偏移量除以li高度。获取到偏移了几个元素     xxx | 0 向下取整
        let anchorIndex = parseInt(this.touch.anchorIndex) + delta
        this._scrollTo(anchorIndex)
      },
      refresh () {
        this.$refs.listview.refresh()
      },
      scroll (pos) {
        this.scrollY = pos.y
      },
      _scrollTo (index) {
        if (!index && index !== 0) {                                                     // 点击到最上方和最下方的区块，return
          return
        }
        if (index < 0) {
          index = 0
        } else if (index > this.listHeight.length - 2) {                                 // 点击移动到最上方和最下方的区块，index置为最小或最大
          index = this.listHeight.length - 2
        }
        this.scrollY = -this.listHeight[index]                                           // 手动重置scrollY使得相应位置高亮
        this.$refs.listview.scrollToElement(this.$refs.listGroup[index], 0)
      },
      _calculateHeight () {                                                       // *左右联动，获取每一个阶段的高度
        this.listHeight = []
        const list = this.$refs.listGroup
        let height = 0
        this.listHeight.push(height)
        for (let i = 0; i < list.length; i++) {
          let item = list[i]
          height += item.clientHeight
          this.listHeight.push(height)
        }
      }
    },
    watch: {
      data () {
        setTimeout(() => {                                       // 确保所有手机的兼容。没有使用$nextTick
          this._calculateHeight()
        }, 20)
      },
      scrollY (newY) {
        const listHeight = this.listHeight
        // 当滚动到顶部，newY>0
        if (newY > 0) {
          this.currentIndex = 0
          return
        }
        // 在中间部分滚动
        for (let i = 0; i < listHeight.length - 1; i++) {
          let height1 = listHeight[i]
          let height2 = listHeight[i + 1]
          if (-newY >= height1 && -newY < height2) {
            this.currentIndex = i
            this.diff = height2 + newY
            return
          }
        }
        // 当滚动到底部，且-newY大于最后一个元素的上限
        this.currentIndex = listHeight.length - 2
      },
      diff (newVal) {
        let fixedTop = (newVal > 0 && newVal < TITLE_HEIGHT) ? newVal - TITLE_HEIGHT : 0                  // 向上偏移。是一个负值
        if (this.fixedTop === fixedTop) {
          return
        }
        this.fixedTop = fixedTop
        this.$refs.fixed.style.transform = `translate3d(0,${fixedTop}px,0)`
      }
    }
  }

</script>

<style scoped lang="stylus" rel="stylesheet/stylus">
  @import "~common/stylus/variable"

  .listview
    position: relative
    width: 100%
    height: 100%
    overflow: hidden
    background: $color-background
    .list-group
      padding-bottom: 30px
      .list-group-title
        height: 30px
        line-height: 30px
        padding-left: 20px
        font-size: $font-size-small
        color: $color-text-l
        background: $color-highlight-background
      .list-group-item
        display: flex
        align-items: center
        padding: 20px 0 0 30px
        .avatar
          width: 50px
          height: 50px
          border-radius: 50%
        .name
          margin-left: 20px
          color: $color-text-l
          font-size: $font-size-medium
    .list-shortcut
      position: absolute
      z-index: 30
      right: 0
      top: 50%
      transform: translateY(-50%)
      width: 20px
      padding: 20px 0
      border-radius: 10px
      text-align: center
      background: $color-background-d
      font-family: Helvetica
      .item
        padding: 3px
        line-height: 1
        color: $color-text-l
        font-size: $font-size-small
        &.current
          color: $color-theme
    .list-fixed
      position: absolute
      top: 0
      left: 0
      width: 100%
      .fixed-title
        height: 30px
        line-height: 30px
        padding-left: 20px
        font-size: $font-size-small
        color: $color-text-l
        background: $color-highlight-background
    .loading-container
      position: absolute
      width: 100%
      top: 50%
      transform: translateY(-50%)
</style>
